ORALS

Titanium dioxide (TiO₂) is one of the most attractive materials during the last decades because of its variety of practical applications such as photocatalysis, amphiphilic coatings, and dye-sensitized solar cells. So far, numerous TiO₂ nanostructures have been prepared such as spheres, films, nanowires, nanofibers, and nanotubes. In addition to the trend of nanostructural fabrication, the microarchitecture of TiO₂ has become a new paradigm in recent materials, chemistry, and nanotechnology [1]. Ammonium oxofluorotitanate (NH₄TiOF₃) crystals have been intensively studied and used as precursors for the synthesis of highly ordered and morphologically controlled TiO₂ [2]. It has been found that co-doping of nonmetal (especially with N and F) and metal (Fe) elements is capable of extending the light absorption edge of TiO₂ to the visible light region and improving photocatalytic activity due to the synergistic effect [3, 4]. For the large-scale applications of such materials, their mass quantity is required. In this case, the fluoride processing of the most abundant natural mineral ilmenite (FeTiO₃) with solid NH₄HF₂ (m. p. 126 ^oC) can be used. We have carefully studied this process [5]. Fluorination reactions of raw materials with NH₄HF₂ are thermodynamically possible and proceed exothermally (even at room temperature). Fluorination products are mainly nonstoichiometric phases of high symmetry: tetragonal double salts of silicon and titanium (NH₄)₃[Si(Ti)F₆]F, cubic fluoroelpasolites and fluoroperovskites, which were isolated in a single crystal form and their crystal structures were determined (or refined). Their phase transitions at temperature decreasing, thermal and hydrolytic properties were studied. The compounds were investigated by X-ray diffraction, differential thermal analysis (DTA), differential scanning microcalorimetry (DSM) and adiabatic calorimetry, NMR, infrared, Raman, XPS, and Mössbauer spectroscopies. The tensimetry method was also used for establishment of incongruent sublimation of (NH₄)₂SiF₆. Comparative analysis of fluoride processing methods of titanium-containing raw materials indicates the preference of NH₄HF₂ use.

References:
[1] H.K. Lee, S.W. Lee, Chem. Lett. 44 (2015) 604-606.
[2] H.K. Lee, T. Fujiwara, T. Okada, T. Fukushima, S. W. Lee, Chem. Lett. 47 (2018) 628-631.
[3] P. Zhang, M. Fujitsuka, T. Majima, Appl. Catalysis B, 185 (2016) 181-188.
[4] Y.F. Zhang, H.Y. Shen, Y.H. Liu, Res. Chem. Intermed. 42 (2016) 6265-6287.
[5] N.M. Laptash, I.G. Maslennikova, Adv. Mater. Phys. Chem. 2 (2012) 21-24.